1. Field of the Invention
The present invention concerns methods and devices to set shim parameters of a magnetic resonance apparatus to prepare the implementation of a magnetic resonance examination of a patient having an imaging medical magnetic resonance apparatus having a displaceable patient bed.
2. Description of the Prior Art
Magnetic resonance tomography is known from, for example, DE 102005019859 A1; “Floating Table Isotropic Projection (FLIPR) Acquisition: A Time-Resolved 3D Method for Extended Field-of-View MRI During Continuous Table Motion,”. Fain, et al., Magnetic Resonance in Medicine 52: 1093-1 102 (2004); “Continuous Adjustment of Calibration Values for Improved Image Quality in Continuously Moving Table Imaging”, Shankaranarayananl et al., Proc. Intl. Soc. Mag. Reson. Med. 11 (2004); U.S. Pat. No. 5,523,688; and “Helical MR: Continuously Moving Table Axial Imaging With Radial Acquisitions”, Shankaranarayanan et al., Magnetic Resonance in Medicine 50: 1053-1060 (2003).
One goal of magnetic resonance tomography is to implement comprehensive patient examinations with increasingly shorter and stronger basic field magnets. The problem thereby occurs when larger body regions (the entire patient or non-contiguous regions) must be examined with an increasingly smaller imaging region due to the shorter basic field magnet. The magnetic resonance apparatuses (MR apparatuses) with short magnets, which are advantageous with regard to interventional questions and with regard to increased patient comfort, thereby pose new requirements for the workflow of an examination with such a magnetic resonance apparatus. The problem of the examination of an examination region that cannot be acquired with a local acquisition is solved by movement of the patient through the imaging region by means of a displaceable patient bed. The displacement can ensue in steps or continuously. In the former case, conventional, local acquisition techniques can be used.
However, the basic field of the magnet is altered by the patient depending on the location currently occupied by the patient due to movement of the patient bed. The field is thus altered differently at different positions of the patient bed, and therefore different positions of the patient located thereon. The basic field of the magnet is designated as B0; its variation is designated as field inhomogeneity.
The basic field should be homogenized by shimming (shim currents in shim magnets and gradient offsets) for the current position of the patient bed, and therefore of the patient located thereon.
For this purpose, the inhomogeneous basic magnetic field can be measured at the current position of the patient bed and the patient.
The measurement ensues as, for example, a gradient double echo imaging sequence in which the phase of the image is directly proportional to the field inhomogeneity. For a stationary measurement, the shim measurement is typically a 3D gradient echo sequence that runs for approximately 20 seconds. The shim parameters (shim settings) are only valid for a path of approximately 30 mm of the patient bed.
For an image acquisition given a continuous movement of the table, it is hardly practical to implement the conventional static measurement over a larger region in a spatially sufficient narrow grid. Particularly for oncological applications, the fat saturation is significant and this requires a good shim.
For a series of stationary measurements at many different, freely selectable positions of the patient bed, the frequent repetitions of stationary shim measurements likewise mean a marked extension of the total measurement time. Only static measurements at regular intervals of patient bed positions have previously been proposed for imaging with continuous table displacement (see the aforementioned article by Fain et al.).